The present invention is generally related to seismic sensors, and more particularly to fiber optic seismic sensor systems. As used throughout this application and its appended claims, seismic movement and activity can mean any vibrations capable of measurement by a land-based sensor, whether caused by geologic activity, other natural phenomena, explosions, the motion or effects of mechanical implements or any other activity causing vibrations in a land mass.
The traditional method for detecting seismic signals has been the coil-type geophone. Geophone sensors comprise a mass-spring assembly contained in a cartridge about 3 cm long and weighing about 75 grams. In a typical geophone sensor, the spring is soft and as the cartridge case moves the mass (coil) is held in place by its own inertia. Thus, the coil acts as a reference for measurement of the cartridge displacement. This sensor arrangement is used for measurement of large, oscillatory displacements on the order of millimeters with sub-micrometer resolution. The frequency range of these sensors is limited, however. For best sensitivity to small displacements, a given sensor has a mechanical bandwidth of about 10 Hz. Sensors can be designed with center frequencies from 20 Hz to 100 Hz.
Micro-Electro-Mechanical Systems (MEMS) are miniature mechanical components fabricated in silicon wafers. The fabrication methods are based on the same photolithographic and etching processes used to manufacture electronic circuits in silicon. In fact, most MEMS devices include not only miniature mechanical components such as beams, nozzles, gears, etc. but also, integrated electronic components to provide local signal conditioning. Unfortunately, the integrated circuits limit the maximum operating temperature of electronic MEMS to 75° C. The maximum temperature limit can be extended to 400° C. or more if optical fiber sensors are integrated with mechanical MEMS components so that no electronics are needed in the high temperature environment.
Recently, MEMS accelerometers have been developed for 3-component (3C) land seismic measurements. In the MEMS accelerometer, a mass-spring assembly is also used, but unlike the geophone, the spring is stiff and the mass moves with the case that houses the MEMS. The inertia of the mass causes strain and deflection of the spring and the deflection or strain can be measured with a sensor to determine the acceleration. High performance 3C MEMS accelerometers with capacitance sensors have been demonstrated.
The measurement range of accelerometers is specified in units of ‘G’ where 1G=9.8 m/s2. Commercial specifications include 120 dBV dynamic range (1G to 10−6 G) and 500 Hz mechanical bandwidth with 24-bit digital resolution equivalent to a noise limited performance of 10−7G/(Hz)1/2. The accelerometer is fabricated on a silicon chip on the order of 100 mm2 and weighing roughly 1 gram. Three single-axis accelerometers (each with an application specific integrated circuit (ASIC) on each chip for signal conditioning) are packaged to measure in three orthogonal directions. The limitation of these accelerometers is an upper limit on the operating temperature of 75° C., which is imposed by the electronic integrated circuits and is not a fundamental limitation of silicon itself.
Additional objects and advantages are set forth in the description which follows, as well as other that may be obvious from the description, known to those skilled in the art or may be learned by practice of the invention.